The following correction was printed in the Guardian's Corrections and clarifications column, Saturday July 10 2004

We say below that Reiter's syndrome, also known as reactive arthritis, is "a condition caused by untreated gonorrhea". That is not its only cause. According to the Arthritis Foundation, the exact cause is unknown, but about 75% of the those with the tendency to develop it have a special gene marker. Reiter's can develop following an infection in the intestines or genital or urinary tract. See www.arthritis.org for more details.

In her navy blue track suit, Jasmine crosses her legs and shifts her gym bag more comfortably on to her lap. She sits opposite Dr Emma Fox, the consultant who runs Bridge, a pioneering sexual health clinic for under-20s at Guy's Hospital in south London. Jasmine explains tentatively that her contraceptive pill has been causing irregular bleeding and she doesn't know why. Fox notes her symptoms and runs through a list of questions. "Is this a regular sexual partner?"; "Is he from another country?"; "Do you use condoms?"; "Have you had any other sexual partners?"

At the last question, Jasmine gives a wobbly smile. This is her first boyfriend and she was sexually inexperienced before they got together. Fox assesses that her patient is low-risk for HIV but suggests running a screen for syphilis and gonorrhea because either of these infections could explain the bleeding. Her next patient, another young woman, had unprotected sex with a stranger in March and is concerned that she might have put herself at risk of HIV or chlamydia. She just wants to be "totally safe", and Fox refers her for testing in another room.

The need for a clinic aimed at young people could not be more stark. Britain has the worst sexual health in Europe and the boroughs of Lambeth, Southwark and Lewisham, in south London, have the highest rates of sexually transmitted infections (STIs) in the country. The incidence of chlamydia has risen by 50% in teenagers in the past three years alone. Fox, who has headed Bridge since its opening in February this year, says an estimated 15% of her patients have this infection, which can be asymptomatic and lead to infertility in women if untreated. These south London boroughs also have a quarter of all UK cases of gonorrhea.

"Having more services isn't the answer to improving teenage sexual health," says Fox. "The government has got the right idea with strategies that recognise the link between poverty, social exclusion and poor sexual health, but changing things is actually quite difficult." The young people who attend Bridge, however, are fortunate because they can avoid the queues and waiting lists that affect genito-urinary medicine clinics throughout the country. As STI rates soar, with cases of chlamydia having increased by 140% between 1996 and 2002, and HIV diagnoses by 200%, clinics are burdened with huge increases in their caseloads.

Even worse, as patients are turned away from overcrowded clinics before they can be diagnosed or treated, they run the risk of infecting others; studies suggest that approximately a third of patients with symptomatic STIs continue to have sexual intercourse. The consequences can be devastating. Rebecca, 23, slept with a former boyfriend who claimed he hadn't been with anyone else since their split. "He wanted to break it off so he could have told me anything and I would have had him back," she says. "We slept together and then I started having really painful cramping, even when it wasn't my period." Rebecca's test proved positive for chlamydia and after a course of antibiotics, she got the all-clear.

Rebecca regards herself as fortunate because an untreated STI can lead to chronic illness or permanent damage. "It can take a lot of courage to turn up to one of these places and if you're told to come back in six weeks, that person might not come back," says Jan Barlow, chief executive of the Brook advisory service. "We certainly hear cases of people being turned away at GUM clinics." Although the problem is more acute for young people, with women under 20 having the highest rates of gonorrhea and chlamydia, all sexual health services are currently under pressure.

Bill, 37, describes his experience of visiting a walk-in clinic in east London that was so crowded, people were sitting on the floors and in the corridors. A year ago, Bill, suspecting the symptoms of an infection, headed to a clinic near work for treatment and found the door locked. "I got there in good time but found a sign saying, 'Due to too many patients, we have closed the clinic early,' " he says. "The trouble was that I was going to eastern Europe for a 10-day holiday so I couldn't just come back the next day." A few days into his trip, Bill developed a raging sore throat and took an antibiotic to clear up the symptoms. "I knew I was in deep shit," he says. "But what do you do? I was moving around and wasn't in one place so I foolishly decided to treat myself."

The symptoms cleared up. But a few weeks later, Bill began to have painful swelling in his tendons, chest pains and was becoming breathless. "The doctors kept telling me they didn't think anything was wrong with me and wanted to prescribe anti-depressants." A few months later, he developed stiff joints but was told he was too young to have arthritis and it was nearly a year before he was finally diagnosed with Reiters syndrome, a condition caused by untreated gonorrhea.

Bill admits that he had unprotected sex while he was on holiday, a mistake that Ann, aged 29, also made. "Earlier this year I split up with my husband and went on holiday to Spain. It was the usual thing of meeting a guy, having had too much to drink and I didn't think anything of it until I got home and talked to a girlfriend." Although Ann had no symptoms of an infection, her friend suggested she get tested. So, one lunchtime, she went to a walk-in clinic. She was shocked to discover that she had tested positive for chlamydia. "I would never make the same mistake again," she says. "Some people think it only affects stupid 16 year-olds but it affects everyone."

In fact, the risk of contracting an STI has increased as people have more partners and engage in riskier sexual behaviour. "Condom use has increased but it hasn't kept pace with the unsafe sex that people are having,' says Dr Patrick French, who runs the Mortimer Market Centre clinic at University College, London. "People are having sex at a younger age and there are more overlapping relationships and partners. Young people are crying out for more support about the message out there that starting sex early is what's expected."

Sexual health professionals all seem to agree that destigmatising sexual health and starting sex education at an early age are vitally important to curbing the dramatic rise in infections. With the government's planning and priority framework for health expected early next month, the Terrence Higgins Trust is lobbying to make sexual health a priority while the Independent Advisory Group on Seuxal Health and HIV is pushing for work on prevention.

There are cutting-edge projects already operating in Lambeth. Dr Claire Gerada, a local GP in south London, has been visiting primary schools in the borough for the past five years, teaching children how to negotiate relationships with their peers, how to resist peer pressure and what changes their bodies go through during puberty. Gerada says she often visits schools where the detailed questions the children ask suggest they are already sexually active.

In a year-six class on a Monday morning, a mixed class of girls and boys sit quietly, their hands folded, their desks neat. Gerada is asking the children what they know about puberty. They eagerly raise their hands, desperate to give answers. "Boys get sperm", "Sometimes your voice breaks", "Girls' hips widen". One girl asks how far a penis can travel up a vagina and another asks if you can get pregnant before you've started your periods. When Gerada asks why condoms are important, a girl in plaits says calmly, "Because it stops spreading diseases."

Afterwards the children tell me confidently that they think these lessons, which they started in reception, will affect the choices they make once they leave primary school. "I really do think it will change the way I think about sex and relationships in high school," says Emma, 11. "I won't just jump straight into things." Her classmate Lawrence agrees. "These lessons are preparing us for secondary school because if we talk about sexual relationships now, we have an idea of what's going to happen there. It's a good start for us."

French, who may have these children walking through his clinic doors in a few years, agrees that the key to combating STIs lies in early education. "In countries where children get sexual and relationship education from age seven, they start having sex later, they're much more grown up and responsible," he says. "Now we're at a time when people are more open, we need to keep pushing on that open door."